1. Field of the Invention
One disclosed aspect of the embodiments relates to solid-state imaging apparatuses used for scanners, video cameras, digital still cameras, and so on.
2. Description of the Related Art
In recent years, image sensors having a built-in analog-digital (AD) conversion circuit have been studied. As one configuration of such an image sensor, there is a type called column ADC in which an AD conversion circuit (ADC) is provided for each column of a pixel array. Further, a ramp-type column ADC is well known as one type of AD conversion employed for the column ADC. The ramp-type column ADC system includes a comparison unit provided for each column and a ramp signal source. In general, the comparison unit compares a ramp signal with a pixel signal, while the ramp signal serves as a reference signal. Further, an amount of elapsed time until the relationship between the pixel signal and the ramp signal in terms of the potential has been reversed is measured, and the measured amount of elapsed time is stored as digital data in a column memory that is disposed on each column. For example, Japanese Patent Application Laid-Open No. 2009-60327 discusses a method for using a reference signal (i.e., slope signal) and a counter known as a common counter which counts time that has passed after voltage of the reference signal starts a change from its initial voltage.
In the image sensor including the ramp-type column ADC using the common counter, if there is a large difference in potential between the ramp signal and the pixel signal, comparison processing is not completed within a predetermined AD conversion period, thus data is not written in the storage unit. In such a case, the storage unit stores the data that has been written previously. This may cause abnormal images to be output. Japanese Patent Application Laid-Open No. 2006-81203 discusses a method for suppressing the above problem by adding a level fixing circuit. However, with the method discussed in Japanese Patent Application Laid-Open No. 2006-81203, there is a problem in that a chip area may be increased by adding the level fixing circuit.